Predation Risk and the Importance of Cover for Juvenile Rainbow Trout in Lentic Systems

Juvenile rainbow trout Oncorhynchus mykiss stocked in midelevation reservoirs in Utah inhabit shallow nearshore areas and are vulnerable to predation from piscivorous fish and birds. We determined the use and importance of nearshore habitats by (1) measuring habitat selection by, prey availability to, and feeding of juvenile rainbow trout in two reservoirs with populations of predators, and (2) measuring survival and growth rates in the presence and absence of cover in a pond experiment where adult brown trout Salmo trutta were predators. In the reservoirs juvenile rainbow trout (60–120 mm standard length) were abundant in complex inshore habitats but avoided simple habitats such as sand and gravel during the day. At night, however, juvenile rainbow trout in both reservoirs were observed in more exposed areas, and they rested on the bottom. Measurements of gut fullness indicated that juvenile rainbow trout fed actively on large Daphnia spp. during the day but little at night. Because large daphnia were usually more abundant offshore than inshore in both reservoirs, selection of inshore cover is believed to be primarily a response to greater predation risk offshore. In the pond experiment, the presence of brown trout significantly increased mortality of juvenile rainbow trout, decreased their growth rates, and caused them to avoid open-water areas. The presence of cover significantly decreased predation rates but did not affect growth of the juvenile rainbow trout. Fisheries managers should consider augmenting cover in reservoirs and lakes where juvenile trout are stocked to minimize losses of trout to predators